1. Field of the Invention
The present invention generally relates to an image processing apparatus for medical diagnosis and, more particularly, to an apparatus for correcting defects in images, or for enhancing specific image portions by spatial shift variant filtering.
2. Description of the Prior Art
Many image processing techniques have been proposed in the medical-electronic field in, for instance, U.S. Pat. No. 4,334,244 to Chan et al. and U.K. Patent Application GB 2020945A to Anthony et al.
When considering the image processing technique until a single image is obtained in the camera system for displaying the image, filtering is executed in order to correct for deterioration of image quality of the resultant image due to distortions and blurring caused by the optical system and the camera imaging system. Further, such filtering may be also intentionally executed on the basis of the personal sense so as to compensate for such a drawback as much as possible that the image information to be enhanced cannot be sufficiently perceived by the human sense of sight.
When considering picture elements (pixels) constructing a single image, this image has a great amount of information. Different resolution often simultaneously exists in the same image, for example, in the peripheral and central portions of the image. That is to say, the imaged portions of an object under medical examination on a single image have different resolution from each other.
Further, there is also a case where the imaging system has the different frequency characteristics in the peripheral and central portions of the imager. Particularly, such a problem occurs in the medical examination television apparatus or the like. Namely, describing in detail, in the case of using, e.g., a television camera, in the peripheral portion of the photosensitive layer of the pickup tube, image distortions occur at a higher ratio than that in the central portion of this photosensitive layer. It is known that this phenomenon is one of the inherent drawbacks which are caused when an electron beam scans on the photosensitive layer. The pincushion distortion is the typical one of those distortions. In addition, there is also such a problem that the distortion which is caused due to the image intensifier tube is also superimposed on the distortions.
As the conventional effective means for eliminating the above drawbacks in the image processing techniques, the following method has been used. Namely, an original image, i.e., non-processed image obtained from the system (hereinafter, this image is simply referred to as an original image) is subjected to spatial filtering to improve the spatial frequency characteristics of the image itself and thereby to improve the information which is obtained from the original image. However, the image cannot be satisfactorily corrected merely by executing the spatial shift invariant-filtering depending on the features of the imaging apparatus to obtain the original image and of the original image itself. Therefore, it is demanded that the original image is subjected to the spatial shift variant-filtering according to the image characteristic at a high speed and the image is sufficiently corrected and the characteristic portions are extracted.
The image processing technique using a conventional space invariant filter will then be simply explained hereinbelow.
Namely, consideration is made with respect to an example such that with regard to the memory in which image information is stored for every pixel, for example, the first pixel and the second pixel which is fairly apart therefrom are filtered by the low-pass filters having the filter coefficients of the same value.
However, according to such a conventional invariant-filtering, only the filtering of the same state can be performed with respect to each pixel. In the above example, there exists a fact that there is actually the case where it is necessary to inherently make different the correction and enhancement of the images since the first pixel is spatially away from the second pixel. In spite of this fact, in this case, there is a drawback in that only the image processings of the same state can be executed.